Getting exception after running pipeline: "can't convert from BSON type String to Date"

Question

I am getting an issue

"can't convert from BSON type String to Date"

for running the following aggregate query:

db.test.aggregate(

    // Pipeline
    [
        // Stage 1
        {
            $match: {"Total Number Of CM": { $exists: true }}
        },

        // Stage 2
        {
            $project: {
                  _id : 0,
                  Date: { $dateToString: { format: "%Y-%m-%d", date: "$CollectionTime" } },
                  DeviceName: 1,
                  ResourceName: 1,
                  "Total Number Of CM": 1,
                  "Online Number Of CM": 1,
                  "Offline Number Of CM": 1,
            }
        },

        // Stage 3
        {
            $group: {
                "_id": { Date: "$Date"},
                ResourceName: { $first: "$ResourceName" },
                DeviceName: { $first: "$DeviceName" },
                TotalCM: { $avg: "$Total Number Of CM" },
                OnlineCM: { $avg: "$Online Number Of CM" },
                offlineCM: { $avg: "$offline Number Of CM" }

            }
        }
    ]
);

Please, I need help to solve this issue. The sample looks like:

{
    "DeviceName" : "OLT0",
    "ResourceName" : "OLT/Frame:1/Slot:1/Port:0",
    "CollectionTime" : ISODate("2016-11-03T13:35:00.000+01:00"),
    "GranularityPeriod" : NumberLong("5"),
    "Total Number Of CM" : NumberLong("26"),
    "Online Number Of CM" : NumberLong("22"),
    "Offline Number Of CM" : NumberLong("4")
}